Phase and frequency control of a radio frequency generator from an external source

ABSTRACT

Controlling a phase and/or a frequency of a RF generator. The RF generator includes a power source, a sensor, and a sensor signal processing unit. The sensor signal processing unit is coupled to the power source and to the sensor. The sensor signal processing unit controls the phase and/or the frequency of a RF generator.

FIELD OF THE INVENTION

The present invention generally relates to a radio frequency (RF)generator and, more particularly, to phase and frequency control of a RFgenerator.

BACKGROUND OF THE INVENTION

Plasma etching is frequently used in semiconductor fabrication. Inplasma etching, ions are accelerated by an electric field to etchexposed surfaces on a substrate. To effectively execute a plasma etchingprocess, the power delivered from a radio frequency (RF) generator tothe plasma chamber must be precisely controlled. An example of an RFgenerator for use in an RF plasma system may be found with respect toU.S. Pat. No. 6,707,255, issued on Mar. 16, 2004, the disclosure ofwhich is incorporated by reference in its entirety herein.

A RF generator system typically includes a RF power source, a RF sensor,and a sensor signal processing unit. The RF power source generates RFpower signals detected by the sensor. The sensor outputs analog sensorsignals representative of the RF power signals. The analog sensorsignals include a frequency of interest and a plurality of interferingfrequency components. The sensor signal processing unit band limits theanalog sensor signal to remove interfering frequency components from theanalog sensor signal, thereby leaving an analog or digitalrepresentation of the frequency of interest.

It is desirable to accurately control the frequency and the phase of RFgenerators. For example, the typical phase offset for fixed or agilefrequency of RF generators is not controlled. Phase values derived froma linear combination of the RF sensor signals also are typically notadequately controlled. Moreover, conventional RF generators do notexternally control a variable phase offset, such as in an agilefrequency RF auto tuning frequency system while maintaining a variablefrequency offset between the RF generators.

SUMMARY OF THE INVENTION

The present invention is directed to a radio frequency (RF) generator.The RF generator includes a power source coupled to a sensor. A sensorsignal processing unit is coupled to the power source and to the sensor.The sensor signal processing unit is operable to receive an input froman external source. The sensor signal processing unit controls the phaseand the frequency of a RF generator through the external source.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIGS. 1A-1B are block diagrams that depict exemplary radio frequency(RF) systems according to some embodiments;

FIG. 2 is a block diagram of a sensor signal processing unit in the RFgenerators depicted in FIGS. 1A-1B; and

FIG. 3 is a block diagram of a signal processing architecture accordingto some embodiments.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

The following description of various embodiments is merely exemplary innature and is in no way intended to limit the invention, itsapplication, or uses. For purposes of clarity, similar reference numbersare used in the drawings to identify similar elements.

FIG. 1A depicts a plasma system 10 including a pair of radio frequency(RF) signal generators for driving a mutually exclusive load representedby plasma chamber 20. Plasma system 10 includes first and second RFgenerators 14 a, 14 b, matching networks 18 a, 18 b, and plasma chamber20. RF generators 14 a, 14 b can implement a master-and-slaveconfiguration using a control signal. RF generator 14 a is designatedthe master, and RF generator 14 b is designated the slave. The frequencyof RF generator 14 b may slaved to the frequency of RF generator 14 ausing a control signal sent from RF generator 14 a to RF generator 146.When the control signal is absent, RF generators 14 a and 14 b canoperate autonomously.

RF generators 14 a, 14 b include respective RF power sources 222 a, 222b, RF sensors 224 a, 224 b, and sensor signal processing units 226 a,226 b. RF power sources 222 a, 222 b generate RF power output torespective RF sensors 224 a, 224 b. RF sensors 224 a, 224 b detect theRF power output and generate respective RF power signals that vary inaccordance with the RF power. RF sensors 224 a, 224 b include arespective voltage probe and a current probe. These probes output a RFvoltage signal 202 and a RF current signal 204, as shown in FIG. 2.Alternatively, RF sensors 224 a, 224 b may be directional sensors. Adirectional sensor has outputs that correspond to reflected power orvoltage (REV) and forward power or voltage (FWD).

In some embodiments sensor signal processing units 226 a, 226 b can, forexample, receive and bandlimit the respective signals from RF sensors224 a, 224 b. This removes interfering frequency components and leaves acomponent of interest at a desired frequency. Sensor signal processingunits 226 a, 226 b also control the phase and frequency relationship ofthe RF power for respective RF generators 14 a, 14 b. The RF powersignals are output to respective matching networks 18 a, 18 b. Matchingnetworks 18 a, 18 b match the impedance of plasma chamber 20 to theimpedance expected by first and second RF generators 14 a, 14 b. Thisminimizes reflected power and maximizes power transfer to plasma chamber20. In some embodiments, more than two RF generators 14 a, 14 b can bearranged in the mutually exclusive load configuration of FIG. 1A.

FIG. 1B depicts a plasma system 100 arranged in accordance with someembodiments and including a dual RF signal system for driving a combinedload. Plasma system 100 includes first and second RF generators 14 a, 14b, combiner 160, matching network 180, and plasma chamber 20. First RFgenerator 14 a transmits a control signal to the second RF generator 14b to implement a master-and-slave configuration. RF generators 14 a, 14b are configured as described above with respect to FIG. 1A. Both firstand second RF generators 14 a, 14 b output RF power output to combiner160. Combiner 160 combines the RF power output and generates a single RFpower output to the matching network 180. Matching network 180 matchesthe impedance between the RF configuration components (e.g., RFgenerators 14 a, 14 b and combiner 160) and the impedance of plasmachamber 20. In some embodiments, more than two RF generators 14 a, 14 bcan be arranged in the combined load configuration of FIG. 1B. In someembodiments, more than two RF generators, such as shown in FIGS. 1A and1B, can be daisy chained or paralleled.

FIG. 2 depicts an architecture for implementing a sensor signalprocessing unit 226 a, 226 b of FIGS. 1A and 1B. Sensor signalprocessing unit 226 includes a signal processing module 210 and acontroller 220. Signal processing module 210 includes a plurality of lowpass filters 232 a-c, a plurality of analog-to-digital converters 234,235, a field programmable gate array (FPGA) 236, and a digital signalprocessor 286. The filters described herein are implemented as low passfilters. The filters primarily control the bandwidth of the spectrum ofthe input signal. One skilled in the art will recognize that bandpass orhigh pass filters may be substituted for the low pass filters describedherein. Further, filter 232 c may have specifications differing fromfilters 232 a, 232 b. Low pass filter 232 a receives a RF voltage signal202 output from RF sensor 224, and low pass filter 232 b receives a RFcurrent signal 204 from RF sensor 224. One skilled in the art willrecognize that the selection of the RF metrology sensor 222 a, 222 bdetermines the type of signal input to low pass filters 232 a, 232 b.For example a directional coupler outputs reverse power and forwardpower signals to low pass filters 232 a, 232 b. A voltage/current sensoroutputs voltage and current signals to low pass filters 232 a, 232 bforward power (or voltage) and reverse power (or voltage), respectively,may be input to low pass filters 232 a, 232 b. Low pass filters 232 a,232 b bandlimit the input signal to a frequency of interest received atRF voltage input 202 and RF current input 204, respectively, from sensor224. In some embodiments, anti-aliasing filters implement low passfilters 232 a, 232 b. Low pass filters 232 a, 232 b output respectivesignals to an analog-to-digital (AND) converter pair 234. A/D converterpair 234 operates at a data rate speed sufficient to process theincoming signals. A/D converter pair 234 simultaneously samples thevoltage and current signal output from each low pass filter 232 a, 232b. A/D converter pair 234 converts the analog input signals intocorresponding digital signals output as voltage signal (Vx) and currentsignal (Ix).

Similarly, a phase and frequency input control signal is input to lowpass filter 232 c. The phase and frequency input control signal input tolow pass filter 232 c can contain one or both of phase and frequencyinformation. In some embodiments, sensor signal processing unit 226 isimplemented in a slave RF generator, and the phase and frequencyinformation applied to low pass filter 232 c provides the controlsignal, such as output from RF generator 14 a of FIG. 1 a. In someembodiments, a master RF generator, such as RF generator 14 a of FIG. 1a, may include a sensor signal processing unit 226 as configured in FIG.2. However, in such an embodiment, no phase and frequency information isnot always input to low pass filter 232 c of sensor signal processingunit 226. More specifically, in some embodiments, RF generator 14 afunctions as the master and RF generator 14 b functions as the slave. Itshould be recognized by one skilled in the art, however, that a plasmacontrol system may generate set point information to either or both ofmaster RF generator 14 a and slave RF generator 14 b. The set pointinformation includes at least power level, frequency offset, and phaseoffset information. In addition to the set point information provided bya master plasma control system (not shown), the master RF generatorgenerates the frequency and phase signals from controller 220 to slaveRF generator 14 b, which is input to low pass filter 232 c. Thefrequency and phase signal is generated by the master from a sample ofthe RF power amplifier drive circuit. A splitter provides any necessaryisolation and attenuation. The information input to low pass filter 232c contains an absolute frequency and an absolute phase for defining thebase frequency and phase for operation of the slave RF generator. Thisinformation works in conjunction with the set point information providedby the master plasma control system, namely power level, frequencyoffset, and phase offset.

Low pass filter 232 c may be implemented as an anti-aliasing filter. Thefiltered signal is then input to A/D converter 235. The phase andfrequency input control signal is utilized when the detector circuit isenabled and detects the presence of the input control circuit.Determining the frequency input control signal requires calibration. AU.S. National Institute of Standards & Technology (NIST) traceablefrequency/time standard provides a baseline for performing thiscalibration. A/D converter 235 samples the filtered signal at apredetermined clock rate. In some embodiments, the sampling rates areimplemented through an A/D clock generator (not shown) thatsimultaneously generates an A/D clock signal to A/D converter pair 234and to A/D converter 235. A/D converter 235 generates a digital outputsignal PnFIn representative of the phase and frequency of the inputsignal.

Signals output from A/D converter pair 234 and A/D converter 235 areinput to FGPA 236 or other programmable device for large scaleintegration. FPGA 236 reduces the sample rate of A/D converter pair 234and A/D converter 235 to a data rate sufficient for processing bydigital signal processor (DSP) 286. DSP 286 is adapted to generate acontrol signal 288 that can control the phase and/or frequency of RFgenerators, such as RF generators 14 a, 14 b of FIGS. 1A and 1B. Controlsignal 288 is output to controller 220.

Controller 220 includes direct digital synthesizer (DDS) 290, low passfilter 292, and splitter 295. DDS 290 is a conventional digital circuitconfigured to generate a frequency and phase tunable output signal basedupon input signal 288. DDS 290 outputs a signal to low pass filter 292.In some embodiments, low pass filter 292 is implemented as ananti-aliasing filter. Low pass filter 292 removes spurious frequencycomponents from the signal. The filtered signal output by low passfilter 292 is input to a splitter 295, which is implemented as a powersplitter. Splitter 295 outputs a phase and frequency control signal anda RF drive signal. The phase and frequency control signal controls RFgenerators such as RF generators 14 a, 14 b to vary the phase and/or thefrequency of RF generators. The RF drive signal ensures that the RFgenerators 14 a, 14 b are activated or turned “on”. The RF drive signalis output to matching networks 18 a, 18 b of FIG. 1A or to combiner 160of FIG. 1B.

FIG. 3 depicts an expanded block diagram 300 of a portion of signalprocessing unit 226. FIG. 3 describes the relationship between FPGA 236and DSP 286 of FIG. 2 to control the phase and/or frequency of RFgenerators 14 a, 14 b. Signal processing module 226 includes avoltage/current signal processing unit 310 and a phase/frequencyprocessing unit 320. One skilled in the art will recognize thatvoltage/current processing unit 310 may alternatively be areverse/forward signals from the appropriate RF metrology sensor.Voltage/current signal processing unit 310 removes undesirable frequencycomponents from the frequency of interest, leaving the frequency ofinterest. Voltage/current signal processing unit 310 includes a pair ofmixing modules 344 a, 344 b, a digital frequency synthesizer 342 a, apair of decimation modules 346 a, 346 b, a pair of low pass filters 348a, 348 b, and a pair of coordinate converters 350 a, 350 b.

Digital signals acquired from A/D converter pair 234, shown in FIG. 2,are applied to respective mixing modules 344 a, 344 b. Mixing modules344 a, 344 b also receive two inputs from the digital frequencysynthesizer 342 a. Digital frequency synthesizer 342 a generates mixingsignals. The mixing signals are sine and cosine waveforms representativeof the frequency setpoint. The frequency setpoint correlates to thefrequency of interest. In some embodiments, the frequency setpoint is afrequency offset received from a controller 380 of phase and frequencyprocessing unit 320. Mixing modules 344 a, 344 b combine the mixingsignals with the signals received from A/D converter pair 234. Digitalmixing modules 344 a, 344 b each include two multipliers 345 a, 345 a′;345 b, 345 b′. Mixing modules 344 a and 344 b generate signals with aspectrum constructed from the sum and difference of the frequenciescontained in the sampled input signals and the signals provided bydigital frequency synthesizer 342 a. Translation of the sample rate to amanageable data rate occurs in decimation modules 346 a, 346 b.Decimation is achieved via a cascaded integrator comb (CIC) filter, butother decimation approaches may be implemented to reduce the data rate.One skilled in the art will recognize that the components ofvoltage/current processing unit 310 may be implemented digitally.

Decimation modules 346 a, 346 b generate a reduced data rate signal torespective low pass filters 348 a, 348 b. Low pass filters 348 a, 348 bprovide a shaping function of the spectral output of respectivedecimation modules 346 a, 346 b. In some embodiments, a nineteen taphalfband filter may implement low pass filter 348 a, 348 b. Low passfilters 348 a, 348 b output an in-phase (I) signal and a quadrature (Q)signal for each of the digital power signals input into coordinateconverters 350 a, 350 b, respectively. Coordinate converters 350 a, 350b convert the I and Q signal data for the respective voltages andcurrent input signals from Cartesian to Polar coordinates. Coordinateconverters 350 a, 350 b output phase and magnitude signals for therespective voltage and current input signals. The voltage and currentphase signals are applied to phase and frequency processing unit 320 toenable determination of a phase error signal. One skilled in the artwill recognize that coordinate converters 350 a, 350 b can reside inFPGA 236. In some embodiments the coordinate converters 350 a, 350 b,390 can be consolidated into one coordinate converter. Suchconsolidation digitally synchronizes and temporarily multiplexes thethree complex input signals.

The magnitude signal can be used as a detection mechanism to allow atransfer from internal control of the RF to an external source. Thephase component is used for phase control with respect to the RF sensorphase. The sensor phase is represented by voltage phase and currentphase. The controlled phase of the RF sensor can be any predefinedlinear combination of sum (voltage phase+current phase), difference(voltage phase−current phase), or individual phase value (voltage phaseor current phase). FIG. 3 does not explicitly describe any specific oneof these limited combinations. Voltage and current signals used hereinare used to indicate the output of a voltage and current RF sensor. Thephase component can be used to determine the frequency of the phase andfrequency input control signal, as shown input to low pass filter 232 cof FIG. 2.

Phase and frequency processing unit 320 includes digital mixing module344 c, decimation module 346 c, derivation module 360, controller 380,digital frequency synthesizer 342 b, coordinate transformer 390, andsummers 357, 358, 359, 361. PnFIn input signal, obtained from samplingof A/D converter 235 (shown in FIG. 2), is input to mixing module 344 cof FPGA 236. Mixing module 344 c includes two digital mixers 345 c, 345c′ that receive the PnFIn signal. Mixer 345 c also receives a cosineinput from digital frequency synthesizer 342 b for multiplication withthe PnFIn signal. The PnFIn signal is also multiplied by multiplier 345c′ with a sine signal output by digital frequency synthesizer 342 b. Theproduct output by multiplier 345 a represents the real component (I) ofa complex waveform. The product output by multiplier 345 c′ representsthe imaginary component (Q) of a complex waveform. The complex waveformhas a predefined phase offset. The output from multiplier 345 c containsthe sum of the frequencies of the PnFIn input signal and the DFS 342 bsignal; and the output from multiplier 345 c′ is the difference of thefrequencies of the PnFIn input signal and the DFS signal. Digital mixingmodule 344 c outputs these signals to decimation module 346 c.

Decimation module 346 c reduces the data rate of the two productsignals. The real I and imaginary Q components of the waveform passthrough a digital filter in decimation module 346 c. A CIC filterimplements the digital filter since it is able to efficiently performseveral functions. For example, the bandwidth of the CIC filteraccommodates the offset frequency and allows the control loop to trackthe input frequency before digital frequency synthesizer 342 b isupdated with a new frequency setting through mix frequency error 366.The CIC filter also reduces the incoming sampling rate to a moremanageable sampling rate. The digital filter is connected to acoordinate converter 390.

Coordinate converter 390 converts the filtered complex waveformrepresented by I and Q from Cartesian form to Polar form. Coordinateconverter 390 outputs the magnitude and phase of the PnFIn sample tocontroller 380 and to frequency derivation module 360, respectively. Themagnitude of the PnFIn signal provides a detection mechanism whichallows transfer from an internal control of a RF source to an externalsource. That is, if the magnitude signal output from coordinateconverter 390 is below a threshold, the RF source receiving themagnitude signal is controlled internally. A threshold magnitude isestablished for each type of RF generator 14 a and 14 b to switchbetween internal to external control.

Derivation module 360 determines the frequency, which is the frequencyset point, of the PnFIn signal by determining the derivative of thephase with respect to time. The derivative provides the rate of rotationthrough a complex coordinate system. This rate is directly proportionalto the decimated frequency for the difference between the frequency ofthe sampled input signals and the digital frequency synthesizer 342 b.The derived frequency may also be a filtered version of the derivativeof the sampled input signal with respect to time. For instance, thederivative is calculated with respect to time for the signal and is thenoptionally filtered by a linear (average) or a nonlinear (median)filter. Derivation module 360 outputs the value of the frequency tosummer 361.

Summer 361 sums the output of the frequency derived by the derivationmodule 360 and the input signal desired frequency 288 that the RF systemseeks to achieve. The sum output by summer 361, referred to as frequencyerror 368, is input to controller 380. Controller 380 detects changes inthe frequency of the PnFIn sample using frequency error 368. A frequencyerror 368 of zero indicates that the frequency remains constant. A valueother than zero indicates that the frequency is changing. Controller 380tracks changes in the frequency. Controller 380 also updates digitalfrequency synthesizer 342 a with the DDS frequency and phase offset 392of the input signal. This frequency and phase offset 370 is the desiredfrequency 288 with a frequency offset.

Mix frequency set point 365 and a signal from controller 380 are appliedto summer 359. Mixed frequency set point 365 is an input from anexternal source. In some embodiments, the external source is a userdefined input. Mix frequency set point 365 specifies changes in thefrequency. The sum of summer 359 is the mix frequency error 366, whichis fed back to digital frequency synthesizer 342 b. The mix frequencyerror 366 signal changes the set point to digital frequency synthesizer342 b. In this manner, the frequency of a RF generator 14 a, 14 b iscontrolled.

In some embodiments, an external source controls the phase of RF powergenerators 14 a, 14 b. In addition to being input to derivation module360, the polar representation of the phase of the PnFIn signal also isinput to a summer 357. In the digital domain, summer 357 determines aphase difference as the difference between the phase detected for thePnFIn signal and either the voltage phase (V_(PHASE)) or the currentphase (I_(PHASE)). Optionally, V_(PHASE) and I_(PHASE) may be combinedand then added to summer 357. Alternatively, I_(PHASE) may be subtractedfrom the V_(PHASE). One skilled in the art will recognize that adirectional coupler may be substituted for the voltage/current sensordescribed herein. Accordingly, a reverse signal substitutes for voltageand a forward signal substitutes for current.

Summer 358 receives the phase difference signal output by summer 357 anda phase set point 362. Phase set point 362 is a constraint that controlsthe RF power signal to a certain phase offset. The phase offset may bevaried while the frequency is fixed or agile. Phase set point 362 is aninput from an external source, such as a user defined input. Phase setpoint 362 and phase difference 363 are compared at summer 358 to producea phase error 364. Phase error 364 is the phase offset. Phase error 364is input to controller 380. Controller 380 provides DDS 290 of FIG. 2with frequency and phase data, which controls the RF generators 14 a, 14b.

While one of a number of various embodiments of the system have beendisclosed, various alterations can be made which fall within the scopeof the present invention. For example, RF generators 14 a, 14 b may befixed frequency generators or, alternatively, may be agile frequencygenerators. The agile frequency generator varies the frequency in orderto maximize power transfer from the respective RF generator to plasmachamber 20. The present invention has numerous applications. Forexample, the phase and frequency of RF power signals may be controlledby using a phase lock loop (PLL). The PLL may be implemented in ananalog circuit, a digital circuit, or a combination thereof.Additionally, while the figures depict two RF generators, the presentinvention may be applied to any number of RF generators. For example,controller 220 of FIG. 2 may be replicated in each RF generator. Formultiple RF generators, the signals are daisy chained or paralleled. Inanother of the various embodiments, the digital filter used indecimation process 346 a-c may be a polyphase filter. In still yetanother of the various embodiments, a low pass filter followed by adownsampler may comprise the digital filter.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A radio frequency (RF) generator comprising: a RF power source; asensor coupled to the power source; a sensor signal processing unitcoupled to the power source and to the sensor, the sensor signalprocessing unit adapted to receive an input from an external RFgenerator source and to generate a control signal to control phase andfrequency of the RF generator.
 2. The RF generator of claim 1, whereinthe input is one of a phase set point and a mix frequency set point. 3.The RF generator of claim 2, wherein the external source is a userdefined input.
 4. The RF generator of claim 1, wherein the sensor signalprocessing unit comprises a field programmable gate array (FPGA) unitand a digital signal processing unit.
 5. The RF generator of claim 4,wherein the FPGA unit is operable to receive a phase and frequency inputsignal.
 6. The RF generator of claim 4, wherein the signal processingunit is operable to generate the control signal.
 7. The RF generator ofclaim 6, wherein the signal processing unit comprises a controllercoupled to a derivative function component.
 8. The RF generator of claim7, wherein the controller generates the control signal and wherein thecontrol signal is a RF signal.
 9. The RF generator of claim 8, whereinthe control signal generated from the controller depends on a pluralityof inputs to the controller.
 10. The RF generator of claim 9, whereinthe plurality of inputs include a phase error signal, a frequency errorsignal, and a phase and frequency input signal.
 11. A radio frequency(RF) system comprising: a first agile frequency RF generator including:a power source; a sensor coupled to the power source; and a sensorsignal processing unit coupled to the power source and to the sensor,the sensor signal processing unit adapted to receive an input from anexternal source and to generate a first control signal controlling atleast one of a first phase and a first frequency of a RF power signalfrom the first RF generator; and a second agile frequency RF generatoroperable to receive a second control signal from the first RF generator,the second control signal controls at least one of a second phase and asecond frequency of the second RF generator.
 12. The RF generator ofclaim 11, wherein a variable frequency offset between the first and thesecond RF generators is controlled by the second control signal, andwherein the first and second control signals are RF signals.
 13. The RFgenerator of claim 11, wherein the input is a user defined input.
 14. ARF system comprising: a first RF generator; and a second RF generatorcoupled to the first RF generator, the first and the second RFgenerators each including: a power source; a sensor coupled to the powersource; a sensor signal processing unit coupled to the power source andto the sensor, wherein the sensor signal processing unit for the firstRF generator controls a phase and a frequency of the first RF generatorand the sensor signal processing unit for the second RF generatorcontrol a phase and a frequency of the second RF generator.
 15. The RFsystem of claim 14, wherein each sensor signal processing unit controlsa variable phase offset for a fixed frequency.
 16. The RF system ofclaim 14, wherein each sensor signal processing unit controls a variablephase offset for an agile frequency.
 17. The RF system of claim 15,wherein a variable frequency between the first and the second RFgenerators is maintained through a control signal from the first RFgenerator to the second RF generator.
 18. A method comprising: couplinga sensor to a power source; coupling a sensor signal processing unit tothe power source and to the sensor; receiving an input from an externalsource; and generating a control signal to control phase and frequencyof a RF generator.
 19. The method of claim 18, wherein the input is oneof a phase set point and a mix frequency set point.
 20. The method ofclaim 18, wherein the external source is a user defined input.
 21. Themethod of claim 18, wherein generating the control signal comprises:taking a derivative of a phase and frequency signal; calculatingfrequency of a signal based on the derivative; summing a frequencyoffset with the derivative; updating a frequency error at a controller;and sending a mixed frequency error to a digital frequency synthesizerfrom the controller.
 22. The method of claim 21 further comprising:receiving a phase offset from an external source; generating a variablephase setpoint; and controlling the phase of the RF generator.